1. Field of The Invention
The invention relates to a fuel-cell element which comprises a combination of two porous and metallic electrodes with a gastight and oxidic electrolyte layer, and a process for its manufacture.
2. Description of The Related Art
Fuel cells are interesting devices for converting chemical energy directly into electrical energy. Theoretically, substantially higher conversion rates are achievable than via the Carnot cycle process. A very simple hydrogen/oxygen cell was put forward as long as 150 years ago by R. Grove. Since that time the problem has been to react fuel (for example CH.sub.4) electrochemically with oxygen (air) in such a way that the energy of oxidation of carbon monoxide to CO.sub.2 and of the hydrogen component to H.sub.2 O is directly converted into electrical energy. In this connection, the following reactions occur at the cell electrodes: ##STR1##
This requires a gastight separation of the coreactants by means of a barrier, the electrolyte layer, which is permeable only to oxygen ions but is electrically insulating. Electrical connection has in turn to be made with both sides of said barrier in such a way that the supply and removal of the electron currents is ensured in as loss-free a manner as possible. At the same time, good accessibility of the electrolyte surfaces for the gaseous coreactants (CO and H.sub.2 on the anode side, O.sub.2 on the cathode side) and removal of the reaction products (H.sub.2 O vapor and CO.sub.2) are required.
For most fuels, carbon monoxide and hydrogen have first to be produced via preliminary thermal reactions (reforming), for example for CH.sub.4 via the reactions: ##STR2## For the invention described below, it is irrelevant whether the starting gases for the electrochemical reaction are produced simultaneously inside the fuel cell or outside and upstream of it, or whether pure hydrogen is directly converted into electrical energy.
At this point it should also be stated that the process can also be reversed with the fuel cell element according to the invention. This relates, in particular, to the electrochemical decomposition of H.sub.2 O vapor into H.sub.2 and O.sub.2, or of other gaseous oxides, for the purpose of fuel synthesis accompanied by power consumption.
Many types of fuel cell have already been proposed. In various versions, an attempt has been made to overcome the discrepancy between the theoretical conversion rates and the values achieved in practice. Real electrochemical reaction rates are limited by mass transport, by electron conduction, by the physical properties of the cell element materials used and by geometrical effects. A key role is played by the two porous electrodes, whose structural and material long-term stability has a very critical effect on the performance of the cell element.
Among the many proposed cells, high-temperature fuel cells with solid electrolyte are of particular interest. The application temperatures of this cell type are between 700.degree. and 1100.degree. C. They can therefore be fed directly with air and natural gas or other gaseous hydrocarbons. A cell temperature which is adequate for an internal reforming process for CO is desirable. In addition to electricity, usable heat can also be generated simultaneously by a chemical combustion, a very important factor for the total energy balance. An adjustable proportion of the fuel is burnt, and the entire cell is thereby simultaneously kept at a temperature level which is favorable for the electrochemical conversion.
The solid electrolyte preferably used for this type of cell is cubic stabilized ZrO.sub.2, a material which conducts oxygen ions at fairly high temperatures with very good electrical resistance for electron conduction.
Since an individual element only generates a low electrical voltage, many individual elements are assembled to form batteries. The individual cells have to be electrically connected in series with one another, using a material which conducts electrons very well and has as low a contact resistance as possible. Inside the cell battery, air and fuel must be able to flow through the individual elements in a manner which is optimum for the system, with strict gastight separation of the two coreactants. Such "cell stacks" furthermore need external supply and removal lines for current and gases. Particular problems exist for high-temperature cell stacks since the essential component, namely the solid electrolyte, is a ceramic material. Despite very careful matching of materials and production methods, it has hitherto not been possible to withstand in the longer term the mechanical loadings always arising during operation because of thermal gradients and different coefficients of expansion. Individual electrolyte elements break under the mechanical stresses produced, the gastightness is gradually lost and the efficiency drops. Rising temperatures resulting from the chemical fuel reaction destroy the cell stack.
Although small batteries were initially operated successfully, all the stacks produced in the 70s have been abandoned. Because of new ceramics technology, better production possibilities are now emerging without, however, being able to overcome the conflicting properties relating to cell physics and cell mechanics: U.S. Pat. No. 3,460,991 describes, for example, a solid-electrolyte cell battery in tubular form. A similar construction is described by Westinghouse, 1979, in a publication entitled "Thin Solid Fuel Cell/Battery Power Generation System", a porous ceramic tube being used as the actual support structure. Using various coating methods, the porous cathode, the dense electrolyte layer, the porous anode and additional electrically conducting and electrically insulating layers are built up one after another. In Japan, too, a tubular battery having an output power of 1 kW was successfully built and tested. As described in a publication of the Electrochemical Laboratory, Ibaraki 305, Japan, an attempt was made to build up at least all the cell elements with the aid of the thermal spraying technique instead of various coating methods. The Japanese solution also differs from the American one in that porous Al.sub.2 O.sub.3 tubes are used instead of ZrO.sub.2 ceramic. Simultaneously, the Japanese attempted to produce a plurality of cells in series on one ceramic tube, whereas each ceramic tube is only one cell element in the Westinghouse model. Both designs have the disadvantage that deficiencies have to be borne in the cell performance because of inadequate gastightness.
In order to increase the compactness of the electrolyte and simultaneously raise the packing density compared with batteries built up from tubes, solutions using planar technology have been proposed. In a publication issued by Argonne National Laboratory, 1983, entitled "Advanced Fuel Cell Development," a monolithic design is described with honeycomb openings for the gas transport planes. In this case, pressed and sintered electrolyte structures are joined to the metallic electrode plates with the aid of high-temperature fusion, a design which implies all the mechanical difficulties. Attempts to make advances in this connection are to be found in U.S. Pat. No. 4,721,556. A very compact cell battery is produced by using self-supporting thin electrolyte plates which have been manufactured by plasma spraying under controlled deposition conditions. However, the required gastightness can only be ensured by means of high-temperature sintering, in which process a flattening of the plates and a surface smoothing are simultaneously achieved. The porous electrodes are then applied by the flame spraying process and a plurality of such cell elements is supplemented with the aid of structured metal plates (interconnector) to form batteries. Contrasted with the advantage of an optimum packing density is the disadvantage of extreme mechanical stress loading if gastightness is to be guaranteed in this sandwich construction. The cell construction impedes the gas flows. Furthermore, individual element testing before producing the battery is not possible. And if an individual element is malfunctioning during operation, repair is not possible.
The object of the present invention is therefore to provide a high-temperature solid-electrolyte fuel cell of the type described at the outset which does not have the disadvantages mentioned, is simple and inexpensive to manufacture, can be tested individually before connection in series and can be linked to form cell stacks without mechanical stresses endangering the long-term operation.